var Base64 = {
   _keyStr: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",
   encode (str) { // 加密
     var output = "";
     var chr1, chr2, chr3, enc1, enc2, enc3, enc4;
     var i = 0;
     str = this._utf16to8(str);
     while (i < str.length) {
       chr1 = str.charCodeAt(i++);
       chr2 = str.charCodeAt(i++);
       chr3 = str.charCodeAt(i++);
       enc1 = chr1 >> 2;
       enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
       enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
       enc4 = chr3 & 63;
       if (isNaN(chr2)) {
         enc3 = enc4 = 64;
       } else if (isNaN(chr3)) {
         enc4 = 64;
       }
       output = output + this._keyStr.charAt(enc1) + this._keyStr.charAt(enc2) + this._keyStr.charAt(enc3) + this._keyStr.charAt(enc4);
     } return output;
   },
   decode (input) { // 解密
     var output = "";
     var chr1, chr2, chr3;
     var enc1, enc2, enc3, enc4;
     var i = 0;
     input = input.replace(/[^A-Za-z0-9\+\/\=]/g, "");
     while (i < input.length) {
         enc1 = this._keyStr.indexOf(input.charAt(i++));
         enc2 = this._keyStr.indexOf(input.charAt(i++));
         enc3 = this._keyStr.indexOf(input.charAt(i++));
         enc4 = this._keyStr.indexOf(input.charAt(i++));
         chr1 = (enc1 << 2) | (enc2 >> 4);
         chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
         chr3 = ((enc3 & 3) << 6) | enc4;
         output = output + String.fromCharCode(chr1);
         if (enc3 != 64) {
             output = output + String.fromCharCode(chr2);
         }
         if (enc4 != 64) {
             output = output + String.fromCharCode(chr3);
         }
     } return this._utf8to16(output);
   },
   _utf16to8: function(str) {
     var out, i, len, c;
     out = "";
     len = str.length;
     for(i = 0; i < len; i++) {
         c = str.charCodeAt(i);
         if ((c >= 0x0001) && (c <= 0x007F)) {
             out += str.charAt(i);
         } else if (c > 0x07FF) {
             out += String.fromCharCode(0xE0 | ((c >> 12) & 0x0F));
             out += String.fromCharCode(0x80 | ((c >> 6) & 0x3F));
             out += String.fromCharCode(0x80 | ((c >> 0) & 0x3F));
         } else {
             out += String.fromCharCode(0xC0 | ((c >> 6) & 0x1F));
             out += String.fromCharCode(0x80 | ((c >> 0) & 0x3F));
         }
     }
     return out;
   },
   _utf8to16 (str) { 
     var out, i, len, c;
     var char2, char3;
     out = "";
     len = str.length;
     i = 0;
     while(i < len) {
         c = str.charCodeAt(i++);
         switch(c >> 4)
         {
             case 0: case 1: case 2: case 3: case 4: case 5: case 6:case7:
               out += str.charAt(i-1);
             break;
             case 12: case 13:
               char2 = str.charCodeAt(i++);
               out += String.fromCharCode(((c & 0x1F) << 6) | (char2&0x3F));
             break;
             case 14:
               char2 = str.charCodeAt(i++);
               char3 = str.charCodeAt(i++);
               out += String.fromCharCode(((c & 0x0F) << 12) |
                 ((char2 & 0x3F) << 6) |
                 ((char3 & 0x3F) << 0));
             break;
         }
     } return out;
   }
 }
 
 /* 暴露函数 */
 module.exports = Base64